Maharashtra Board Class 11 Chemistry Chapter 15 Hydrocarbons PDF Download

15. Hydrocarbons

The compounds which contain carbon and hydrogen as the only elements are called hydrocarbons.

Can You Recall?

1. What are hydrocarbons?

2. Write structural formulae of the following compounds: propane, ethyne, cyclobutane, ethene, benzene.

Hydrocarbons can be open chain or cyclic. In accordance with presence or absence of carbon-carbon multiple bond (C=C and/or C≡C) the hydrocarbons are called unsaturated or saturated. Cyclic unsaturated hydrocarbon can be either aromatic or nonaromatic.

Do You Know?

Why are alkanes called paraffins?

Alkanes contain only carbon-carbon and carbon-hydrogen single covalent bonds. They are chemically less reactive and do not have much affinity for other chemicals. Hence they are called paraffins.

Figure 15.1 shows the Classification of hydrocarbon:

Hydrocarbon divides into two main groups: Open chain (aliphatic) and Cyclic.

Open chain (aliphatic) contains: Saturated (alkane) which has Straight chain and Branched chain; and Unsaturated (alkene, alkyne) which has Straight chain and Branched chain.

Cyclic contains: Saturated (alicyclic) which has Aromatic and Nonaromatic; and Unsaturated.

15.1 Alkanes

Alkanes are aliphatic saturated hydrocarbons containing carbon-carbon and carbon-hydrogen single covalent bonds. You have learnt in Chapter 14 that alkanes have general formula \(C_nH_{2n+2}\) where 'n' stands for the number of carbon atoms in the alkane molecule.

15.1.1 Isomerism in Alkanes

Alkanes with more than three carbon atoms generally exhibit structural isomerism and in particular, the chain isomerism.

Figure 15.2 shows IUPAC and common names of structural isomers of \(C_4H_{10}\) and \(C_5H_{12}\).

The number of possible structural isomers increase rapidly with the number of carbon atoms.

Teacher's Note

Alkanes are simple molecules made of only carbon and hydrogen atoms joined by single bonds. For example, CNG (Compressed Natural Gas) we use in vehicles is mostly methane, which is an alkane.

Exam Trick

Remember: Alkanes = all single bonds = least reactive = paraffins. Just like oil doesn't mix with water, alkanes also don't react easily with most chemicals.

Points to Remember

Alkanes have the formula \(C_nH_{2n+2}\).
They are also called paraffins because they are unreactive.
Alkanes can be straight chain or branched chain.
They burn in air to produce heat and light.
The first four alkanes are gases at room temperature.

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